The present invention relates to the application of solder to component leads for subsequent surface mounting to a substrate.
Due to advantages such as small component size, reduced board area requirements, decreasing costs and shorter signal paths, surface mount technology is gain wider acceptance in mounting components to substrates. In a typical surface mounting manufacturing process, solder paste is placed on conductive pads or regions of a substrate such as a circuit board using a screen or stencil printing operation. A component is placed on the board with its leads contacting the solder paste. A reflow operation is performed to melt the solder and create a solder bond between each lead and its corresponding conductive region.
As the spacing of leads on circuit components becomes closer, the difficulties in achieving consistent and reliable results with surface mount technology increase. Lead to lead spacings of 0.050 inch are common, and closer spacings such as 0.025 inch are anticipated. Known methods are subject to difficulties including bridging and the consequent short circuiting of solder between adjacent solder joints, open circuits and voids due to insufficient solder in a solder joint and excessive solder at a solder joint interfering with the ability of an installed component to withstand temperature variations.
Such difficulties are much worse when rework is required after initial installation. Rework, i.e., removal and replacement of a component, may be necessary when an original solder joint is defective or when the component is faulty. Initial production techniques such as screen or stencil printing of solder paste may not be practical during rework. As a result, laborious manual methods may be required and satisfactory results may be difficult to attain.
For example, in past rework procedures, the solder joints are melted and the component is removed. The substrate or circuit board mounting region is then cleaned and dressed by removing extraneous solder from the conductive regions of the substrate. Solder paste is then applied, for example, with a syringe. A component to be installed is then positioned and a reflow operation is carried out. In addition to the painstaking nature of the procedure, another difficulty is that the volume of solder paste applied cannot be precisely controlled and variations in the solder joints lead to inconsistencies and problems such as those referred to above.
A different approach has been proposed in which solder material is applied to the component leads rather than to the substrate prior to the surface mounting reflow operation. One such approach is described in an article entitled A NEW SOLDER TRANSFER APPLICATION TECHNIQUE (STAT) FOR COATING LEADS OF APPLIQUE COMPONENTS, International Journal of Hybrid Microelectronics, Vol. 4, #2, pages 14-18, 1981 and in U.S. Pat. No. 4,396,140. In this process, solder is deposited on the flat surface of a solder transfer member or "substrate" in discrete pads corresponding to the component leads. The leads are placed on the solder pads and the solder is reflowed to adhere to the leads. This process is subject to the necessity for accurate measurement and placement of solder pads on the transfer member. If the solder is applied by screen printing, accurate stencil alignment is required each time the process is performed. With stencil printing, the volume of solder that can be present in a solder pad is inherently limited. Also of concern is bridging between unrestrained solder pads during reflow, especially when lead to lead spacing is close or the quantity of solder is large.